17350
2005, 109, 17350-17355 Published on Web 08/25/2005
Microwave Synthesis of Supported Au and Pd Nanoparticle Catalysts for CO Oxidation Garry Glaspell, Lindsay Fuoco, and M. Samy El-Shall* Department of Chemistry, Virginia Commonwealth UniVersity, Richmond, Virginia 21284-2006 ReceiVed: May 22, 2005; In Final Form: July 28, 2005
We report the microwave synthesis and characterization of Au and Pd nanoparticle catalysts supported on CeO2, CuO, and ZnO nanoparticles for CO oxidation. The results indicate that supported Au/CeO2 catalysts exhibit excellent activity for low-temperature CO oxidation. The Pd/CeO2 catalyst shows a uniform dispersion of Pd nanoparticles with a narrow size distribution within the ceria support. A remarkable enhancement of the catalytic activity is observed and directly correlated with the change in the morphology of the supported catalyst and the efficient dispersion of the active metal on the support achieved by using capping agents during the microwave synthesis. The significance of the current method lies mainly in its simplicity, flexibility, and the control of the different factors that determine the activity of the nanoparticle catalysts.
Nanophase metal and metal oxide catalysts, with controlled particle size and shape, exhibit high surface area and densely populated unsaturated surface coordination sites that can result in significantly improved catalytic performance over conventional catalysts.1-4 The large number of surface and edge atoms provide active sites for catalyzing surface reactions. Research in this area is motivated by the possibility of designing nanostructured catalysts that possess novel catalytic properties such as low-temperature activity, selectivity, stability, and resistance to poisoning and degradation effects.1 Such catalysts are essential for technological advances in environmental protection, improving indoor air quality, and in chemical synthesis and processing. Among the current important environmental issues is the lowtemperature oxidation of carbon monoxide, since small exposure (ppm) to this odorless, invisible gas can be lethal.5 Therefore, there is a need to develop highly active CO oxidation catalysts to remove even a small amount of CO from the local environment. It has been demonstrated that nanoparticles of precious metals such as Au, Pd, and Pt, when used as CO oxidation catalysts, are not as susceptible to moisture and sulfur-containing compounds which typically affect the performance of transition metal oxide catalysts.6,7 Haruta and co-workers demonstrated that the high surface area exhibited via Au nanoparticles makes them particularly useful for the catalytic oxidation of CO to CO2.8,9 The high activity of the Au catalysts is consistent with the strong tendency of Au nanoparticles to efficiently adsorb CO molecules. Surprisingly, the Au nanoparticles do not strongly adsorb and activate oxygen molecules.10 Thus, it is now well-accepted that the oxide support plays a key factor in the activation of oxygen molecules during the CO oxidation.11-13 In this letter, we report a simple method to prepare Au and Pd nanoparticle catalysts supported on CeO2, CuO, and ZnO and compare their catalytic activities for CO oxidation. We also demonstrate that the shape and morphology of the support nanoparticles can have a significant effect on the activity of the catalyst. The approach utilized in the present work is based 10.1021/jp0526849 CCC: $30.25
on microwave synthesis of nanoparticles from metal salts in solutions. Microwave irradiation (MWI) has several advantages over conventional methods, including short reaction time, small particle size, narrow size distribution, and high purity.14-18 Synthesis of the nanoparticles of CeO2, CuO, or ZnO was achieved by dissolving approximately 4 g of Ce(NO3)4, Zn(NO3)2, or Cu(NO3)2 (Alfa Aesar), respectively, in ethanol. While stirring, 10 N NaOH (Alfa Aesar) was added dropwise until the pH of the resulting solution was 10. The resulting solution was then placed in a conventional microwave. The microwave power was set to 33% of 650 W and operated in 30-s cycles (on for 10 s, off for 20 s) for 10 min. The resulting powder was washed with distilled water and ethanol and left to dry. M-doped oxide support nanoparticles (M ) Au or Pd) were prepared as above, but with the addition of the appropriate amounts of the metal salt (HAuCl4 or Pd(NO3)2) mixed with the Ce(NO3)4, Zn(NO3)2, or Cu(NO3)2 solution to obtain the desired dopant concentration (2%, 5%, or 10%). For capped nanoparticles, the starting precursors were mixed with poly(ethylene glycol) (PEG, molecular weight ) 1450) or poly(Nvinyl-2-pyrrolidone) (PVP, molecular weight ) 40 000) as a protective polymer prior to microwaving. The X-ray diffraction (XRD) patterns of the powder samples were measured at room temperature with an X’Pert Philips Materials Research Diffractometer, with CuKR radiation. The samples were mounted on a silicon plate for X-ray measurements. For the CO catalytic oxidation, the sample was placed inside a Thermolyne 2100 programmable tube furnace reactor. The sample temperature was measured by a thermocouple placed near the sample. In a typical experiment, 4 wt % CO and 20 wt % O2 in He was passed over the sample while the temperature was ramped. The gas mixture was set to flow over the sample at a rate of 100 cm3/min controlled via MKS digital flow meters. The conversion of CO to CO2 was monitored using an infrared gas analyzer (ACS, Automated Custom Systems, Inc.). All the catalytic activities were measured (using 20 mg sample) after a heat treatment of the catalyst at 300 °C in the reactant gas mixture for 15 min in order to remove moisture and adsorbed © 2005 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 109, No. 37, 2005 17351
Figure 1. (A) TEM of CeO2 nanoparticles prepared via microwave irradiation (the scale bar is 50 nm). (B) XRD of the CeO2 nanoparticles as prepared (top) and after the oxidation of CO (bottom). (C) Temperature dependence for the CO conversion over: (blue) bulk CeO2 (Aldrich, particle size
